As a circuit member in electric and electronic apparatus there is normally used a flexible circuit board. In some cases, a self-shape retaining circuit member obtained by bending a rigid circuit board into a predetermined shape may be used.
For example, as a circuit member provided between a battery and an apparatus driving portion there has heretofore been used a flexible printed circuit board. It has been a common practice that a metal tab as a contact point to apparatus is mounted on the terminal of the flexible circuit substrate on which electronic parts are mounted and the metal tab is connected to the apparatus driving portion. However, the mounting of the metal tab on the flexible circuit board can cause thermal shock that deteriorates the reliability of the circuit board. In recent years, an attempt has been made to use a self-shape retaining circuit board obtained by stamping a conductor circuit integrated with a metal tab out of a metal foil, interposing the circuit board between rigid plastic films with an adhesive layer to form a composite laminate, and then bending the composite laminate under press.
Such a self-shape retaining circuit member needs to comprise a tough rigid plastic film, i.e., plastic film having an elastic modulus of not less than 450 kg/mm to withstand press molding and assure stable self-shape retention.
As well known, when heated, a plastic film tends to shrink due to residual stress or recrystallization during film forming.
A circuit member is a composite of a metal foil with a plastic film. Assuming that the percent thermal shrinkage, elastic modulus and thickness of the plastic film are .zeta., Ep and tp, respectively, and the elastic modulus and thickness of the metal foil are Em and tm, respectively, the percent thermal shrinkage X of the circuit member is given by the following equation: EQU X=.zeta./(1+Em.multidot.tm/Ep.multidot.tp)
Since a conventional flexible printed circuit board comprises a plastic film having a small elastic modulus of Ep and a small thickness of tp, its entire percent thermal shrinkage is small as evident from the foregoing equation.
However, the foregoing self-shape retaining circuit member needs to comprise a rigid plastic film having a high elastic modulus and a large thickness of tp to withstand press molding and assure stable self-shape retention. Thus, Ep and tp cannot be reduced, and the resulting X value is increased. Therefore, the thermal expansion and shrinkage of the foregoing self-shape retaining circuit board itself cannot be neglected. The resulting thermal stress can break the point at which it is connected to apparatus or battery or the point on which an electronic part is mounted. Accordingly, assured excellent reliability can hardly be given to the foregoing self-shape retaining circuit member under severe thermal conditions.
It is therefore a first object of the present invention to provide a self-shape retaining rigid circuit member shaped into a predetermined shape by bending under press, which exhibits assured press-moldability and self-shape retention as well as assured reliability against severe thermal use.
In the assembly of electric and electronic apparatus, the connection of a circuit portion comprising a rigid circuit board to other circuit portions or power supply may be made via a flexible circuit board.
As such a flexible circuit board there has heretofore been normally used one comprising a polyethylene terephthalate film or polyimide film as a substrate or cover.
The connection of a rigid circuit board to a flexible circuit board has traditionally been accomplished by soldering. In recent years, in order to meet the demand for high precision in conductor on circuit board, it has been attempted to use an anisotropic electrically-conductive film.
An attempt has been made as follows. In some detail, an anisotropic electrically-conductive film, which comprises a film-shaped adhesive having a thickness of scores of microns and containing electrically-conductive particles incorporated therein such that both ends of the particle are exposed out of both sides of the film and the adhesive is interposed among the particles, is interposed between surfaces to be connected. The laminate is then heated under pressure so that the film-shaped adhesive undergoes flow deformation to connect the conductors on the surfaces to be connected through the electrically-conductive particles and fill the gap between the surfaces to be connected with the adhesive.
This connection process makes it possible to connect high precision circuits having, e.g., 20 conductors per mm, that is, conductor pitch of 50 .mu.m.
In the case where two members a, b are superposed and connected to each other as mentioned above, if the two members a, b shrink at different expansion and shrinkage rates, stress is produced at the interface of the two members. In this case, taking the expansion and shrinkage rate, thickness and Young's modulus of the member a as Ka, Ta and Ea, respectively, and taking the expansion and shrinkage rate, thickness and Young's modulus of the member b as Kb, Tb and Eb, respectively, the resulting X is given by the following equation (1): EQU X=(Ka-Kb)/[1/(Ta.multidot.Ea)+1/(Tb.multidot.Eb)] (1)
In order to reduce (Ka-Kb) in the equation (1) and hence reduce the resulting thermal stress with respect to heat cycle applied to the point at which the foregoing glass circuit board b and plastic flexible circuit board a are connected, a plastic substrate having a small thermal expansion coefficient can be used for the flexible circuit board to advantage.
A plastic expands or shrinks either when it absorbs moisture or it drys. Thus, stress is generated at the foregoing connecting portion either when the plastic substrate in the flexible circuit board absorbs moisture or it drys as evident from the equation (1)
Fatigue failure of the connecting portion on circuit board due to stress caused by thermal expansion and shrinkage has heretofore been considered problematical. However, stress caused by moisture absorption or drying has not too much been considered problematical from the standpoint of fatigue failure.
However, if the glass circuit board exhibits substantially zero hygroscopic and drying expansion and shrinkage rate and the rigid circuit board is such a glass circuit board, assuming that Kb in the equation (1), which is the hygroscopic and drying expansion and shrinkage rate of the glass circuit board, is 0 and Ka is the hygroscopic and drying expansion rate of the plastic flexible circuit board, the resulting stress Y is given by the following equation (2): EQU Y=Ka/[1/(Ta.multidot.Ea)+1/(Tb.multidot.Eb)] (2)
Thus, the hygroscopic and drying expansion and shrinkage rate Ka of the plastic flexible circuit board is straightly reflected in the stress. The resulting stress cannot be neglected. The fatigue and damage caused by the stress cannot be neglected.
Among the foregoing flexible circuit boards, the circuit board comprising a polyethylene terephthalate film as a substrate exhibits a high thermal expansion and shrinkage rate and thus is not appropriate from the standpoint of prevention of fatigue and damage on the foregoing connecting portion due to stress. The circuit board comprising a polyimide film as a substrate exhibits a small thermal expansion rate but a high hygroscopic and drying expansion and shrinkage rate and thus is not appropriate from the standpoint of prevention of fatigue and damage on the foregoing connecting portion due to stress.
It is therefore a second object of the present invention to provide a plastic circuit board adapted to be superposed on and connected to the terminal of a circuit board having a substantially zero hygroscopic and drying expansion and shrinkage rate such as glass substrate, which comprises a plastic substrate that is appropriate for the prevention of fatigue and damage due to stress on the connecting portion, taking into account the fact that not only thermal expansion and shrinkage rate but also hygroscopic and drying expansion and shrinkage rate take great part in fatigue and damage on the connecting portion.
Further, electric and electronic apparatus comprise many printed circuit boards incorporated therein. A printed circuit board is formed by integrating an electrical insulating substrate film and a conductor circuit with an adhesive into a laminate. In general, it is formed by laminating two sheets of substrate films with a conductor circuit provided interposed therebetween with an adhesive. Alternatively, it may be formed by alternatingly laminating a plurality of conductor circuits and a plurality of substrate films with an adhesive provided interposed therebetween.
As such a substrate film there has heretofore been often used a polyethylene terephthalate film or polyimide film.
In recent years, electronics has made remarkable progress. With the enhancement of the density of electric and electronic apparatus and the reduction of the size, thickness and weight of electric and electronic apparatus, printed circuit boards having higher density have been required for these apparatus, and the enhancement of the dimensional stability of circuit pattern has been keenly desired more and more. In particular, under high humidity conditions, printed circuit boards absorb water to show a dimensional change that impairs its connection to other electronic parts such as connector. Thus, the enhancement of the reliability of printed circuit boards against humidity has been desired.
Further, in order to assist global environmental protection, it has been required to recycle resources from printed circuit boards, too.
In order to meet the foregoing requirements for dimensional stability and humidity reliability of printed circuit boards and recyclability of materials from circuit boards, it is necessary that an optimum substrate film and adhesive composition be selected to design printed circuit board. The process for design of such printed circuit boards needs to involve due consideration of various physical properties of the substrate film, occasionally the effect of the composition of the adhesive on the properties of the printed circuit board. In particular, the optimization of physical properties of the substrate film is important for the satisfaction of the requirements for printed circuit boards.
However, none of polyimide films or polyethylene terephthalate films which have been used as substrate films can satisfy all the requirements for printed circuit boards. Further, JP-A-8-130368 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes the use of a polyethylene naphthalate film as a substrate film. However, an ordinary polyethylene naphthalate film is disadvantageous in that it exhibits physical properties close to that of polyethylene terephthalate film and thus shows a great dimensional change under high temperature conditions.
The inventors noted that the film to be used as a substrate needs to exhibit a high elastic modulus and a small thermal expansion coefficient in order to give a good dimensional stability under high temperature conditions during the production and actual use of printed circuit board, needs to exhibit a small hygroscopic expansion coefficient, a small water vapor permeability and a small percent water absorption in order to give a high dimensional stability even under high humidity conditions and needs to exhibit a low melting point in order that the conductor circuit can be easily separated by heating and melting the substrate film when the materials are recycled from waste printed circuit boards.
Then, the inventors made extensive studies of the effect of the physical properties of various substrate film materials on the properties of printed circuit boards. As a result, it was found that a polyethylene naphthalate film having an elastic modulus of not less than 500 kg/mm.sup.2, thermal expansion coefficient of not more than 1.5.times.10.sup.-5 /.degree. C., a hygroscopic expansion coefficient of not more than 1.2.times.10.sup.-5 /%RH, a water vapor permeability of not more than 15 g/m.sup.2 /mil.multidot.day, a percent water absorption of not more than 2% and a melting point of not higher than 280.degree. C. is optimum as a substrate film for printed circuit board. Thus, the present invention has been worked out.
It is therefore a third object of the present invention to provide a printed circuit board comprising the foregoing specific polyethylene naphthalate film.